The present invention relates to a technique for determining appropriate luminance linearization of gray levels for sub-pixel positioning tasks. Additionally, the present invention relates to a technique for optimizing grayscale characters for specific display devices.
Traditional characters are analog in nature, their
shapes defined by smoothly-varying boundaries. With the advent of raster-scan displays and printers, the analog letterforms--produced by optical and mechanical methods--have been replaced with digital representations which can only approximate their predecessors. This will always be the case since character edges have frequencies of infinite magnitude that can never be exactly reproduced with discrete devices. On the other hand, since the visual system is band-limited, it is only necessary to match the quality of the transmitter (i.e., the display device) to the capabilities of the receiver (i.e., the visual system). Unfortunately, the available resolution of most current display devices pale in comparison to the resolving power of the visual system. Furthermore, pixel point spread functions in display devices usually differ substantially from the ideal reconstruction kernel (i.e., the sync function).
An alternative to higher resolution is the use of grayscale technology, where in addition to black and white pixels, a multitude of gray levels are realizable. In general, if each pixel is represented with n bits, 2.sup.n different grayscales are available to each pixel (subject to possible limitations of the display technology). Using gray pixels at the edges of characters can achieve a more faithful representation of the master character than any bi-level version could on the same grayscale device.
Until recently, most text on raster displays used characters represented as binary matrices, the ones and zeros corresponding to the black and white dots to be displayed. Typically, only one set of characters was provided, simple and tuned to the characteristics of the display. Lately, grayscale technology has allowed the incorporation of gray pixels in the character description, leading to a perceived quality improvement when comparing the discrete version of a character with its analog predecessor. With the advent of higher-resolution bi-level displays as well as grayscale devices, there is more flexibility in font sizes and styles which are achievable, but techniques still need to be developed to aid in the production and evaluation of such fonts.
Numerous factors contribute to the perceived quality of digital characters displayed on raster-scan devices such as cathode ray tubes. Due to the characteristic differences between the various display technologies, it is not possible to design a single set of characters that will have acceptable image quality on all devices. Quite often, the only approach to manufacturing suitable character sets for a particular display device is to have a font designer iteratively modify the characters' bitmaps and evaluate them on the screen, until the satisfactory results are obtained. In order to replicate the success of those character matrices, the same type of display must be used and under similar viewing conditions.
Standard filtering techniques are commonly used to generate a grayscale character. In this manner, a high-resolution bi-level master character is convolved with a digital filter and sampled to yield lower-resolution grayscale character. A typical grayscale video display system is disclosed, for example, in U.S. Pat. No. 4,158,200 issued June 12, 1979 to Seitz et al. Other examples of grayscale generation are discussed in Warnock, "The Display of Characters Using Gray Level Sample Arrays," Computer Graphics, Vol. 14, No. 3, July 1980, pp. 302-307, and in Kajiya et al, "Filtering High Quality Text for Display on Raster Scan Devices," Computer Graphics, Vol. 15, No. 3, Aug. 1981, pp. 7-15. These references are incorporated herein by reference.
For a particular grayscale display, the spatial resolution and number of intensity levels available is predetermined for the grayscale character generation process. However, many different filters can be used to generate a character. Furthermore, even with a single filter, different versions of the same character can be generated by shifting the sampling grid of the filtered character relative to the origin of the master.
A technique is needed whereby grayscale linearizations may be tailored to the response of the human visual system to grayscale character displays. Additionally, in order to measure character quality objectively and effectively, font designers need automated tools for both character generation and image-quality evaluation. Although many systems have been developed for generating characters, utilities for evaluating them are sorely lacking. Accordingly, there is a need for systems which may be used to generate and evaluate high-quality grayscale characters.